U.S. Pat. No. 5,554,203 discloses a filter element of the general kind defined above. The filter element comprises a main stiff support frame including the frame itself and traversing bars or ribs which define filter pocket openings. The main support frame is built of an exterior subframe that snugly fits over an interior subframe. The edges of the filter pockets are inter-engaged by opposite subframe wall portions. The interfitting subframes are detachably connected one to another and to the filter pockets. On the outer periphery of the interior subframe there are a number of protrusions which can be snappingly locked into corresponding receiving openings provided in the outer periphery of the exterior subframe, enabling the interlocking engagement between the subframes to be easily unlocked for replacement of the filter pockets. In a preferred embodiment, the traversing bars or ribs of the subframes are also locked to each other by releasable security means, such as so-called magnetically operated security means.